ND Filter Calculator
Convert your metered shutter speed to the time needed behind an ND filter, by filter name, stops or optical density, with stacked filters and a bulb timer.
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About this toolWhat is the ND Filter Calculator?
A dark neutral density filter makes the viewfinder black and the camera meter unreliable, so long-exposure photographers meter without it and then multiply. This ND filter calculator does that step. Enter the shutter speed you measured without the filter, choose the filter by its name (ND8, ND1000), its strength in stops or its optical density (0.9, 3.0), and read the new exposure time.
You can stack two filters, switch to film to apply a reciprocity exponent, and start a bulb countdown for the exact time. A second panel works backwards: say how long you want the shutter open and it tells you how many stops you need and which stock filter comes closest.
- Filter by ND number, stops or optical density
- Two-filter stacking with stops and factors combined
- Optional film reciprocity power-law correction
- Bulb countdown timer for the computed time
- Reverse mode: stops and filter needed for a target time
- Table of your exposure behind common filters
How to use the ND Filter Calculator
- Compose and focus without the filter, then meter or take a correct test shot and note the shutter speed.
- Enter that shutter speed, for example 1/125, 1/30 or 0.5.
- Choose how the filter is labelled: by name, in stops or as optical density, and add a second filter if you stack them.
- For film, switch to Film and enter the reciprocity exponent from the data sheet.
- Read the new shutter time, switch the camera to Bulb if it is over 30 seconds, and start the timer when you open the shutter.
Worked examples
ND1000 on a bright day
Input
Metered 1/125 s, ND1000 (3.0, 10 stops)
Output
8 s
1/125 × 1000 = 8 seconds, enough to smooth small waves.
Stacked filters for streaking clouds
Input
Metered 1/30 s, ND1000 plus ND8
Output
About 4 min 27 s (12.97 stops)
Factors multiply: 8,000 × 1/30 s = 267 s. Use Bulb and the countdown.
Working backwards
Input
Metered 1/30 s, want 30 s
Output
9.81 stops; closest stock filter ND1000, which gives 33 s
The ratio 30 ÷ (1/30) = 900, and log₂ 900 = 9.81 stops.
Background and insight
An ND filter cuts all visible wavelengths by the same factor. Three labelling systems describe that factor. The ND number is the factor itself: ND8 passes one eighth of the light. Stops are the base-2 logarithm, so ND8 is 3 stops and ND1000 is 9.97 stops (makers round it to 10). Optical density is the base-10 logarithm, so ND8 is 0.9 and ND1000 is 3.0. One stop is 0.301 of density, which is why densities and stops convert with a factor of 0.3.
Because each stop doubles the time, the new shutter time is the metered time × 2^stops. With stacked filters the stops (or densities) add and the factors multiply: a 10-stop and a 3-stop filter together are 13 stops, a factor of 8192. Real filters vary by a third of a stop or so from their label, and strong ones add a colour cast, so a test exposure is worth it.
Film adds one more step. Above about one second most emulsions respond less than the simple reciprocity of time and intensity predicts (Schwarzschild's effect), and makers publish a correction, often as a power law: corrected time = metered time^p. The tool applies that formula when you enter p from your film's data sheet. Digital sensors do not suffer reciprocity failure, so leave it off for digital.
Common use cases
Silky water and misty seas
Turn a fast daylight exposure into several seconds or minutes.
Streaking clouds and empty streets
Use 10 to 15 stops so moving people and clouds blur away over minutes.
Wide apertures in bright light
Keep f/1.8 for shallow focus at midday without running out of shutter speed.
Video with a natural shutter angle
Hold 1/50 s at 25 fps outdoors by cutting the light with a fixed or variable ND.
Tips and limitations
- Meter and focus before screwing on a 10-stop filter; autofocus and the viewfinder struggle behind it.
- Cover the viewfinder eyepiece on a DSLR during long exposures, or stray light can fog the frame.
- Strong filters often add a warm or magenta cast. Shoot raw and set white balance from a grey card or in editing.
- Long exposures heat the sensor. Turn on long-exposure noise reduction or take a dark frame for exposures over a minute.
- Variable ND filters can show an X-shaped dark pattern at their strongest setting, especially on wide lenses.
- If the result is too dark or bright, adjust the time by the stops you see in the histogram rather than starting over.
Frequently asked questions
How many stops is an ND1000 filter?
Log₂(1000) = 9.97 stops, sold as a 10-stop filter with optical density 3.0. It needs exposures 1000 times longer.
What is the difference between ND8, 0.9 and 3 stops?
They are the same filter labelled three ways: a factor of 8, an optical density of 0.9 (log₁₀ 8) and 3 stops (log₂ 8).
How do I calculate exposure time with two ND filters?
Add their stops or densities, or multiply their ND factors, then multiply the metered time by the combined factor. ND1000 plus ND8 is a factor of 8000.
Which ND filter is best for waterfalls?
In shade or overcast light a 3 to 6 stop filter gives the half second to few seconds that suits falling water. In bright sun or for seascapes of a minute or more, use 10 stops.
Do I need reciprocity correction on a digital camera?
No. Digital sensors respond linearly to time. Reciprocity failure affects film above roughly a second, and the correction comes from the film maker's data.
Why is my long exposure darker or brighter than expected?
Filters are often a third of a stop off their label, and the light may change during a long exposure. Check the histogram and adjust by whole or third stops.
Is there an in-depth guide to the ND Filter Calculator?
Yes. “ND Filters for Long Exposure: Stops, ND Numbers and Density” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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